Eucalyptus has been used for centuries to treat fevers and help sterilize wounds. The main ingredient in eucalyptus leaves is eucalyptol, a volatile oil that fights infections and has expectorant properties. As such, eucalyptus oil is employed in the treatment of a vast number of ailments, ranging from bronchitis and asthma to emphysema and chronic snoring. Test-tube research has also shown that eucalyptus fights bacteria such as bacillus subtilis, and several forms of the streptococcus bacteria. This research has yet to be conducted in humans, however. Eucalyptus is also used as an insect repellant in many countries.